1. Field of the Invention
The present invention relates to complementary metal-oxide-semiconductor (CMOS) image sensors, and more particularly, to a CMOS image sensor and a method for fabricating the same, which improves color reproduction by preventing an N type region of a photodiode from adjoining a device isolation film. The CMOS image sensor and the method for fabricating the same also reduces dark current.
2. Discussion of the Related Art
An image sensor is a semiconductor device that converts optical images to electrical signals. The image sensor is classified into a charge-coupled device (CCD) and a CMOS image sensor. The CCD stores charge carriers in MOS capacitors and transfers the charge carriers to the MOS capacitors. The MOS capacitors are approximate to one another. The CMOS image sensor employs a switching mode that sequentially detects outputs of unit pixels using MOS transistors by forming the MOS transistors to correspond to the number of the unit pixels using CMOS technology that uses a control circuit and a signal processing circuit as peripheral circuits.
The CCD has drawbacks in that its driving mode is complicated, power consumption is high, and process steps are complicated due to many mask process steps. Also, it is difficult to integrate signal processing circuits in one chip of the CCD. To solve such drawbacks, a CMOS image sensor based on a method for manufacturing a sub-micron CMOS has been developed.
To display images, the CMOS image sensor sequentially detects signals in a switching mode by forming a photodiode and a MOS transistor in a unit pixel. Also, since the CMOS image sensor uses the CMOS technology, low power consumption is required. Furthermore, the number of masks required is less than the number of masks required for the CCD. For example, the number of masks for a CMOS image sensor is fewer by twenty than the thirty to forty masks required for the CCD. In this way, in the CMOS image sensor, process steps are simplified and various signal processing circuits can be integrated in one chip. Therefore, the CMOS image sensor has received much attention as an image sensor for next generation. The CMOS image sensor is widely used in digital still cameras, PC cameras, mobile cameras, etc.
FIG. 1 is a circuit diagram illustrating a unit pixel of a related art CMOS image sensor. As shown in FIG. 1, the unit pixel of the CMOS image sensor includes a photodiode (PD) 10 and four NMOS transistors. A ground GND is shown. The four NMOS transistors are comprised of a transfer gate (Tx) 30 transferring optical charges collected in the photodiode 10 to a floating diffusion (FD) region 20, a reset gate (Rx) 40 resetting the floating diffusion region 20 by setting the potential of the floating diffusion region 20 at a desired value and discharging the charges, a drive gate (Dx) 50 serving as a source follower buffer amplifier, and a selection gate (Sx) 60 serving as an addresser. A load transistor Vb is formed to allow an output signal to be read outside the unit pixel. Power (VDD) is applied.
FIG. 2 is a sectional view illustrating a layout of the related art CMOS image sensor. As shown in FIG. 2, a pnp type photodiode is provided with a p type epitaxial layer grown on a p+ type substrate. A device isolation film Fox is also provided. An n− type region 70 is formed in the p type epitaxial layer, and a P0 type region 80 is formed on the n− type region 70 and a surface of the p type epitaxial layer.
A reverse bias is applied between the n− type region 70 and the p type region (P0 type region 80 and p type epitaxial layer) so that the n− type region 70 is fully depleted when an impurity ion of the n− type region 70 is properly mixed with an impurity ion of the p type region. Thus, the depletion region is enlarged to the p type epitaxial layer below the n− type region 70 and the P0 type region 80 on the n− type region 70. The depletion region is more abundantly enlarged to the p type epitaxial layer having a relatively low dopant concentration. The depletion region is used for image reproduction because optical charges generated by incident light can be accumulated and stored in the depletion region.
FIG. 3 illustrates absorption coefficient and penetration depth depending on the wavelength of incident light for the related art photodiode. As shown in FIG. 3, in accordance with the increase of wavelength of the incident light to the photodiode, the absorption coefficient decreases constantly while the penetration depth increases constantly. In a related art pixel structure, a blue wavelength has a penetration depth of 0.3 μm and causes a difficulty in the color reproduction of images. By contrast, a red light has a penetration depth of 10 μm. Thus, it is difficult to reproduce respective colors at a ratio of 1:1 in color reproduction using red, green, and blue colors. The failure in obtaining an ideal ratio of 1:1:1 of the respective colors in color reproduction reduces color reproduction.
FIG. 4 illustrates a penetration depth in a related art silicon substrate depending on the wavelength of incident light to the silicon substrate. As shown in FIG. 4, the penetration depth is marked by percentage. A main penetration region of a red light having a wavelength of 700 nm has a penetration depth of 4000 Å to 15000 Å. By contrast, main penetration regions of a blue light and a green light have a penetration depth less than 4000 Å.